If the output signal of an amplifier is treated as a binary signal, the amplifier should have a hard switching characteristic. On the other hand, if the output signal should have a plurality of different values, an amplifier with a linear characteristic should be used. However, the large signal behavior of conventional linear amplifiers has proved not to be satisfactory in certain applications such as videoamplifiers where the output signal must have many different values corresponding to different picture half-tones and must also follow large, rapid changes in the input.
In particular, if a sudden stepwise rise occurs in the input signal of a linear videoamplifier, it is observed that the output signal cannot keep up with the rise of the input signal, because the output voltage of such a videoamplifier is only increased with a limited slew rate. It does not have a hard switching characteristic. Thus, the output signal will reach the desired voltage level only after a certain rise time. If the signal that has to be amplified is a pulse signal which has a pulse width on the order of or even smaller than the rise time of the amplifier, the output signal will not be able to reach its full amplitude. Consequently, the bandwidth of the linear amplifier is limited by this rise time.
A videoamplifier is shown in U.S. Pat. No. 4,414,574. This amplifier is concerned with the problem of cutting off the beam current of a CRT when the input is zero volts. U.S. Pat. No. 3,689,848 shows a voltage-to-current converter circuit having a differential amplifier coupled to the output through transistors in a Darlington configuration. This circuit is designed to simulate a voltage controlled current source with good thermal stability.
It would be desirable, therefore, to provide an amplifier, and more particularly a videoamplifier, which has an increased bandwidth and is capable of reproducing variations in the output level over a wide range of amplitudes, even when the pulse width of the input signal is small. It would also be desirable to have an amplifier which had a linear response to small variations in the input signal and a hard switching response to large variations in the input signal.